1. Field of the Invention
The present invention relates to a light beam scanning optical apparatus, and in particular, to a light beam scanning optical apparatus to be used as image writing means of a laser printer, a digital copying machine or the like.
2. Description of Related Arts
Conventionally, a variety of light beam scanning optical apparatuses have been known as means for writing an image onto a photosensitive member. In those apparatuses, a light beam emitted from a light source unit is scanned as deflected by a polygon mirror and focused on a photosensitive member via an f.theta.-lens or the like.
In recent years, there have been growing demands for increasing the pixel density and increasing the image writing velocity. In compliance with these demands, there has been developed a multi-beam type optical apparatus in which two light sources are provided and light beams emitted from the respective light sources are scanned as deflected in a state in which they are close to each other in a sub-scanning direction so as to write two lines at one scan.
FIG. 14a is a schematic view of a multi-beam scanning optical apparatus which was experimentally produced by the inventors of the present invention. Light beams B.sub.1 and B.sub.2 emitted from laser diodes 2 and 3, which are light sources, are coupled by a beam splitter 4 formed by bonding two prisms via a semitransparent layer 4c so that the light beams B.sub.1 and B.sub.2 travel in the same direction (direction X). Then, the light beams B.sub.1 and B.sub.2 are focused on a photosensitive member (imaging surface) via a lens system. Although the light beams B.sub.1 and B.sub.2 are shown as one line in the figure, they travel close to each other at an interval of several tens of microns in the sub-scanning direction (direction Z).
However, in the optical apparatus shown in FIG. 14a, since the beam splitter 4 is placed on and bonded to a supporting base 100 horizontally, there is a possibility that the position of the beam splitter 4 may fluctuate in the direction of the optical axis X, in the main scanning direction Y or in the sub-scanning direction Z. If the beam splitter 4 fluctuates in the direction of the optical axis X, the light beam B.sub.2, which is emitted from the laser diode 3 and deflected by the semitransparent layer 4c, shifts by .DELTA.Z' on the imaging surface in the sub-scanning direction. In the same way, if the beam splitter 4 rotationally fluctuates on Z-axis, the light beam B.sub.2 shifts by .DELTA.Y' on the imaging surface in the main scanning direction.